In studies on superfused synaptosomes from the rat medulla oblongata, the inhibitory effects of the alpha 2-adrenergic agonist clonidine (0.1 microM) on potassium (15 mM K+) induced 3H-noradrenaline (NA) release was potentiated by 20%, when neuropeptide Y was added to the system. The effect of NPY was detectable at low concentrations (1 nM) and was not dose-dependent. Neuropeptide Y alone produced no significant effects on 3H-NA release. The results may indicate the existence of a presynaptic NPY receptor on the noradrenaline and/or adrenaline nerve terminals, which may enhance the presynaptic alpha 2-adrenoreceptor function to inhibit 3H-NA release.